The use of metal cans has been a cost effective means for packaging and preserving a wide range of products, from chemicals to foods. A common container used for this purpose is the three-piece can, consisting of two ends and a cylindrical body typically fabricated out of thin sheet steel.
High volume can making machinery is used to produce such containers. The body of the can is made by forming a flat sheet of steel into cylindrical shape and then welding the longitudinal joined edges together. Prior to forming the cylindrical body, a protective coating is applied to both sides of the sheet material to inhibit corrosion of the metal from the contents of the can and from the outside environment. The coating also prevents interaction of the metal with the contents, which could result in contamination or spoilage. The protective coating is applied to both sides of the sheet stock, but is held short of the two edges to be welded together which require bare metal-to-metal contact.
In the can body making operation, can machinery forms the cylinder over an arm or mandrel and then welds the side seam. At this stage of the process, a strip of bare metal on both the internal and external sides of the weld remains to be coated in order to provide complete protection on all surfaces. Respective spray guns are typically used to apply a narrow width stripe of protective coating to the area of the welded seam on both inner and outer sides thereof. As the cylindrical can body passes over the welding arm, the internal coating stripe is applied by a spray gun mounted on the end of the mandrel over which the cylindrical can body passes and the external coating stripe is applied to the outside of the weld seam by means of a second externally mounted spray gun.
While various spray guns have been used for this purpose, they have been relatively complex in construction, have been problem prone, and have not leant themselves to easy field service and maintenance. Some spray guns in current use, for example, operate from a source of compressed air. When the gun operates, a stream of coating material is ejected from the nozzle and mixed with air to create a finely atomized spray to coat the weld seam as the can body travels past the spray gun. In some instances, space limitations require a single air supply line to be used for operating the gun and atomizing the liquid. In order to prevent splattering of the coating as it impinges against the can body, a proper air-to-liquid pressure must be maintained. However, the minimum pressure required to operate the on/off mechanism of the gun may not be optimum for atomizing the spray. This can result in the excessive application and splattering of coating material. Air assisted spray guns of such type also employ seals about a movable valve plunger or needle that controls starting and stopping of the liquid spray, and such seals are susceptible to wear and require periodic replacement and maintenance.
Another type of spray gun in current use is a solenoid operated device, using high pressure liquid to coat the welded seam area. No air is needed for atomization since the relatively high pressure difference between the fluid and the atmosphere causes the necessary atomization. However, the volume of spray needed to cover a narrow weld strip area is very small. A problem with high pressure hydraulic atomization is that in order to effect such narrow width spraying a relatively small orifice must be used, which is susceptible to clogging and results in frequent maintenance and downtime of the production line. Solenoid operated spray guns heretofore have not been used for low pressure air-assisted spraying in can manufacturing lines because of the difficulty in directing both liquid and air supplies through the gun while maintaining a streamlined profile sufficient to permit the passage of cylindrical can bodies over the gun. Electrical solenoids also generate heat which can be difficult to dissipate in such restricted environment.
Leakage or other malfunctions in the operating parts of such spray guns also can cause the high speed can manufacturing lines to be shut down while servicing is accomplished. If removal of the gun from the manufacturing line is required, the fluid, air, and/or electric lines must be disconnected and the gun removed from its mountings and replaced. This can be a time consuming and costly procedure.